On Thu, Aug 30, 2018 at 9:43 PM Brooke Clarke <[email protected]> wrote: > I would disagree in that ease of jamming/spoofing is strongly related to > wavelength. That's because antenna efficiency > goes down as the size of the antenna gets smaller than 1/4 wave. > So, it's easy to make a GPS jammer (1,100 to 1,600MHz) since a 1/4 wavelength > is a few inches, something that you can > hold in your hand.
However, the short wavelengths of GPS make beam forming a reasonable countermeasure against jamming. By having a small array of GPS antennas a receiver can digitally form beams that both aim directly at the relevant satellites (so even reducing intersatellite interference) while also steering a deep null in the direction of the jammer. If the jammer is powerful enough to overload the front-end then this won't help, but against a non-targeted area denying jammer it should be fairly effective. There are many papers on GNSS beamforming. ( e.g. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5134596/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5134483/ ) This kind of anti-jamming solution should even be pretty inexpensive -- really no more than the cost of N receivers. Except that it is specialized technology and thus very expensive. :) Seeing some open source software implementing beam-forming was one of the things I hoped to see result from the open hardware multi-band GNSS receivers like the GNSS firehose project ( http://pmonta.com/blog/2017/05/05/gnss-firehose-update/ ) since once you're going through the trouble of running three coherent receivers for three bands, stacking three more of them and locking them to the same clock doesn't seem like a big engineering challenge... and the rest is just DSP work. Even absent fancy beam forming, for GNSS timing with a surveyed position except at high latitudes it should be possible to use a relatively high gain antenna pointed straight up and by doing so blind yourself to terrestrial jammers at a cost of fewer SVs being available. But I've never tried it. In an urban area I noticed my own GPSDOs losing signal multiple times per week. Monitoring with an SDR showed what appeared to be jammers. As others have noted intermittent jamming is pretty benign to a GPSDO. Spoofing, OTOH, can trivially mess up the timing. It's my view that if you need timing for a security critical purpose there isn't really any GNSS based solution commercially available to the general public right now, the best bet is a local atomic reference with a GPSDO used to monitor and initially set it. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
